The present invention relates to a metal coordination compound having a binuclear molecular structure and an organic luminescence device using the metal coordination compound, more particularly to an organic luminescence device exhibiting a long life and a high luminescence efficiency by using the metal coordination compound as a luminescence material.
An extensive study on an organic electroluminescence (EL) device for device formation as a luminescence device of a high-speed responsiveness and a high efficiency, has been conducted.
As described in detail in, e.g., Macromol. Symp. 125, 1-48 (1997), an organic EL device generally has a structure comprising upper and lower two electrodes and a plurality of organic film layers between the electrodes formed on a transparent substrate. Basic structures thereof are shown in FIGS. 1A-1D.
As shown in these figures, an organic EL device generally has a structure comprising a transparent electrode 14, a metal electrode 11, and a plurality of organic film layers therebetween on a transparent substrate 15.
In the device of FIG. 1A, the organic layers comprise a luminescence layer 12 and a hole-transporting layer 13. For the transparent electrode 14, ITO, etc., having a large work function are used, for providing a good hole-injection characteristic from the transparent electrode 14 to the hole-transporting layer 13. For the metal electrode 11, a metal, such as aluminum, magnesium or an alloy of these, having a small work function is used for providing a good electron-injection characteristic to the organic film layers. These electrodes have a thickness of 50-200 nm.
For the luminescence layer 12, aluminum quinolynol complexes (a representative example thereof is Alq3 shown hereinafter), etc., having an electron-transporting characteristic and luminescence characteristic are used. For the hole-transporting layer 13, biphenyldiamine derivatives (a representative example thereof is xcex1-NPD shown hereinafter), etc., having an electron-donative characteristic are used.
The above-structured device has a rectifying characteristic, and when an electric field is applied between the metal electrode 11 as a cathode and the transparent electrode 14 as an anode, electrons are injected from the metal electrode 11 into the luminescence layer 12 and holes are injected from the transparent electrode 15. The injected holes and electrons are recombined within the luminescence layer 12 to form excitons and cause luminescence. At this time, the hole-transporting layer 13 functions as an electron-blocking layer to increase the recombination efficiency at a boundary between the luminescence layer 12 and hole-transporting layer 13, thereby increasing the luminescence efficiency.
Further, in the structure of FIG. 1B, an electron-transporting layer 16 is disposed between the metal electrode 11 and the luminescence layer 12. By separating the luminescence and the electron and hole-transportation to provide a more effective carrier blocking structure, effective luminescence can be performed. For the electron-transporting layer 16, an electron-transporting material, such as an oxidiazole derivative, is used.
Further, in the structure of FIG. 1D, a luminescence layer 12 as a single organic layer is disposed between the metal electrode 12 and the transparent electrode 14. This structure is advantageous in view of productivity of the resultant device, and applicable to production processes using vapor deposition and wet coating. The luminescence layer 12 used in this structure is required to exhibit electron and hole transfer performances in addition to a luminescence performance.
Known luminescence processes used heretofore in organic EL devices include one utilizing an excited singlet state and one utilizing an excited triplet state, and the transition from the former state to the ground state is called xe2x80x9cfluorescencexe2x80x9d and the transition from the latter state to the ground state is called xe2x80x9cphosphorescencexe2x80x9d. And the substances in these excited states are called a singlet exciton and a triplet exciton, respectively.
In most of the organic luminescence devices studied heretofore, fluorescence caused by the transition from the excited singlet state to the ground state, has been utilized. On the other hand, in recent years, devices utilizing phosphorescence via triplet excitons have been studied.
Representative published literature may include:
Article 1: Improved energy transfer in electrophosphorescent device (D. F. O""Brien, et al., Applied Physics Letters, Vol. 74, No. 3, p. 422-(1999)); and
Article 2: Very high-efficiency green organic light-emitting devices based on electrophosphorescence (M. A. Baldo, et al., Applied Physics Letters, Vol. 75, No. 1, p. 4-(1999)).
In these articles, a structure including 4 organic layers devices as shown in FIG. 1C has been principally used, including, from the anode side, a hole-transporting layer 13, a luminescence layer 12, an exciton diffusion-prevention layer 17 and an electron-transporting layer 16. Materials used therein include carrier-transporting materials and phosphorescent materials, of which the names and structures are shown below together with their abbreviations.
Alq3: aluminum quinolinol complex
xcex1-NPD: N4,N4xe2x80x2-di-naphthalene-1-yl-N4,N4xe2x80x2-diphenyl-biphenyl-4,4xe2x80x2-diamine
CBP: 4,4xe2x80x2-N,Nxe2x80x2-dicarbazole-biphenyl
BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
PtOEP: platinum-octaethylporphyrin complex
Ir(ppy)3: iridium-phenylpyrimidine complex 
Such a phosphorescent material is particularly noted at present because it is expected to provide a luminescence efficiency of 100% in principle being four times that of a fluorescent material.
However, such an organic luminescence device utilizing phosphorescence is generally required to be further improved regarding the deterioration of luminescence efficiency and device stability.
The reason of the deterioration has not been fully clarified, but the present inventors consider as follows based on the mechanism of phosphorescence.
Generally, in a phosphorescent material, a life of the triplet excitons is longer by three or more digits than the life of a-singlet exciton. More specifically, molecules are held in a high-energy excited state for a longer period to cause reaction with surrounding materials, polymer formation among the excitons, a change in fine molecular structure, and a change in structure of the surrounding materials.
For this reason, a luminescence center material for use in the phosphorescent-type luminescence device is desired to exhibit a high-efficiency luminescence and a high stability. Further, a phosphorescent material providing a high phosphorescence yield and allowing control of emission wavelength has not been proposed heretofore. Accordingly, such a phosphorescent material is desired to be provided.
In view of the above-mentioned circumstances, an object of the present invention is to provide a phosphorescent material allowing a high phosphorescence yield and control of emission wavelength.
Another object of the present invention is to provide an organic luminescence device using the phosphorescent material capable of producing high-efficiency luminescence and holding a high luminescence for a long period.
According to the present invention, there is provided a metal coordination compound represented by the following formula (1): 
wherein M1 and M2 independently denotes a metal atom selected from the group consisting of Ir, Pt, Rh, Pd, Ru and Os; P is a quadridentate ligand connected to M1 and M1; Q1 is a bidentate ligand connected to M1; Q2 is a bidentate ligand connected to M2; and n is 1 or 2.
In a preferred embodiment, the bidentate ligand Q1 is represented by formula (2) shown below and the bidentate ligand Q2 is represented by formula (3) shown below: 
wherein CyN1 and CyN2 are each cyclic group capable of having a substituent, including a nitrogen atom and bonded to the metal atom M1 or M2 via the nitrogen atom; CyC1 and CyC2 are each cyclic group capable of having a substituent, including a carbon atom and bonded to the metal atom M1 or M2 via the carbon atom with the proviso that the cyclic group CyN1 and the cyclic group CyC1 are bonded to each other via a covalent bond and the cyclic group CyN2 and the cyclic group CyC2 are bonded to each other via covalent bond;
the optional substituent of the cyclic groups is selected from a halogen atom; cyano group; a nitro group; a trialkylsilyl group of which the alkyl groups are independently a linear or branched alkyl group having 1 to 8 carbon atoms; a linear or branched alkyl group having 1 to 20 carbon atoms of which the alkyl group can include one or non-neighboring two or more methylene groups that can be replaced with xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94, and the alkyl group can include a hydrogen atom that can be optionally replaced with a fluorine atom; and an aromatic group capable of having a substituent (that is a halogen atom, a cyano atom, a nitro atom, a linear or branched alkyl group having 1 to 20 carbon atoms of which the alkyl group can include one or non-neighboring two or more methylene groups that can be replaced with xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94, and the alkyl group can include a hydrogen atom that can be optionally replaced with a fluorine atom).
In the above-mentioned formula (1), the quadridentate ligand P may preferably be connected to the metal atoms M1 and M2 each via a carbon atom, an oxygen atom or a nitrogen atom. The metal atom M1 is identical in species to the metal atom M2. The bidentate ligand Q1 may preferably be identical to the bidentate ligand Q2. The bidentate ligands Q1 and Q2 may preferably be respectively a carrier-transporting ligand or an energy-trapping ligand and the quadridentate ligand P may preferably be a luminescent ligand.
According to the present invention, there is also provided an organic luminescence device, comprising: a pair of electrodes disposed on a substrate, and a luminescence layer comprising at least one organic compound disposed between the electrodes, said organic compound comprising at least one species of a metal coordination compound of the formula (1) described above.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.